Photolithography using ultraviolet light is a fundamental technology in the production of semiconductor devices. In the integrated circuit (IC) lithographic process, a photosensitive polymer film is applied to the silicon wafer, dried, and then exposed with the proper geometrical patterns through a photomask to ultraviolet (UV) light or other radiation. After exposure, the wafer is soaked in a solution that develops the images in the photosensitive material. Depending on the type of polymer used, either exposed or nonexposed areas of film are removed in the developing process. The majority of Very Large Scale Integration (VLSI) exposure tools used in IC production are optical systems that use UV light. They are capable of approximately 1 .mu.m resolution .+-.0.5 .mu.m (3.sigma.) registration, and up to 100 exposures per hour; they are commonly operated at 405 nm (so-called H-line) or 436 nm (so-called G-line).
A frequent problem encountered by resists used to process semiconductor devices, is reflectivity back into the resist of the activating radiation by the substrate, especially those containing highly reflecting topographies. Such reflectivity tends to cause standing wave ripples and reflective notches, which interfere with the desired pattern in the photoresist. The notches are particularly bad when the support or metallurgy is non-planar.
The problem is illustrated in FIG. 1, which depicts a substrate 1 on which has been formed a metal pattern 3. The metal has been covered with a polyimide dielectric 5 and the polyimide layer planarized. A resist 7 has been deposited and is being exposed through a mask 9. Incident radiation passes through the apertures in the mask and, in an ideal situation, exposes only those areas 11 directly in line with the apertures. Unfortunately when the metal pattern 3 is highly reflective, as it usually is, the reflected light from the metal and, to a lesser extent, from the substrate impinges on areas of the resist not intended to be exposed.
The art discloses two basic approaches to the problem: (1) change the wavelength of the radiation and (2) incorporate some sort of radiation absorber into or under the photoresist. The first approach is awkward and expensive because it requires a new tool set. The second approach and its addendant drawbacks are illustrated in FIGS. 2 and 3. In FIG. 2 a dye containing a chromophore that absorbs at the appropriate wavelength is incorporated in the resist layer; this cuts down on reflected radiation but also on resist sensitivity. In FIG. 3 a dye containing a chromophore of appropriate absorption is incorporated in a special layer 13 beneath the resist 7; this adds to the process additional steps for the deposition and removal of the layer.
A superior process could be envisioned if it were possible to incorporate a dye into the polyimide layer itself (FIG. 4). While this is a fine idea in theory, in practice it is not straightforward. Since the polyimide will remain as part of the semiconductor device, the modified polyimide layer must be deposited as a normal polyimide layer would be, and then it must survive subsequent curing, planarization and metal deposition cycles in which the temperature exceeds 400.degree. C. Typical UV-absorbing dyes such as curcumin and bixin, when incorporated into polyimide films give rise to dielectric films that are not stable above 300.degree. C. Typical pigments that might be thermostable are insoluble and give rise to problems of homogeneity. Thus there is a need for an antireflective polyimide layer that processes normally and that is extremely thermally stable.